1. Field of the Invention
The present invention generally relates to an optical information recording-and-reproducing unit, and more particularly, to an optical information recording-and-reproducing unit which optically records information to a storage medium, and/or optically reproduces information from the storage medium.
In the specification, the meaning of "recording information" includes "erasing information".
An example of an apparatus including the optical information recording-and-reproducing unit is an optical disk unit. The optical disk unit is usable as a storage unit of a computer system and a file system, etc., since the optical disk unit is suitable for storing programs and a large amount of data. In such an optical disk unit used as the storage unit, as computer systems, etc., are miniaturized, it is desired that the optical system in the optical disk accurately record and reproduce information to improve reliability and to reduce the number of components as much as possible to reduce the overall cost of the optical disk unit.
2. Description of the Related Art
FIG. 1 shows a configuration of a first conventional optical system. The optical system is disclosed in Japanese Laid-Open Patent Application No.1-106341 (FIG. 1). In FIG. 1, in a information side 2 of a storage medium 1, a track 3 including a information area 3a and a middle area 3b is formed. The storage medium 1 rotates about an axis 8 in parallel to an optical axis 00' (discussed later) of the optical system. The information side 2 is scanned by an optical beam b from a laser diode 4. The optical beam b is focused on the information side 2 through a diffraction grating 9 and an object lens system 6 to form a spot V. The optical beam b reflected on the information side 2 is provided to the diffraction grating 9 through the object lens system 6. The optical beam b diffracted in the diffraction grating 9 is provided to an optical detection system 10. In this case, the diffracted optical beam b, from the diffraction grating 9, to the optical detection system 10 is, for example, one of a +1-order diffraction light and a -1-order diffraction light. Based on the +1-order diffraction light or the -1-order diffraction light which are detected in the optical detection system 10, a focus error signal is generated.
FIG. 2 shows a configuration of a second conventional optical system. The second conventional optical system is disclosed in Optical Data Storage Topical Meeting 1992, Technical Digest 32/MC2-MC2/35 (FIG. 3). In FIG. 2, an optical beam from a laser diode 11 is focused on a storage medium (not shown) through a mirror 12, a hologram element 13 and an object lens system (not shown). The optical beam reflected on the storage medium is provided to the hologram element 13 through the object lens system, and is diffracted in the hologram element 13. The diffracted optical beam from the hologram element 13 is provided to an optical detection system 14 including optical detectors 14a to 14d. In this case, the diffracted beam from the hologram element 13 to the optical detection system 14 is both +1-order diffraction light and -1-order diffraction light. Based on the +1-order diffraction light and the -1-order diffraction light which are detected in the optical detection system 14, the focus error signal is generated.
In the first conventional optical system, the optical beam b reflected on the information side 2 is diffracted in the diffraction grating 9 through the object lens system 6, and is provided to the optical detection system 10. The diffracted optical beam from the diffraction grating 9 to the optical detection system 10 is one of the +1-order diffraction light and the -1-order diffraction light. Therefore, for generating the focus error signal, only one of the +1-order diffraction light and the -1-order diffraction light is used. Accordingly, the amount of light used for generating the focus error signal is relatively small. As a result, a current level produced by optical-to-electrical conversion in the optical detection system 10 becomes small, and, thus, there is a problem that the quality of the detected signal is degraded.
Therefore, to improve availability of the +1-order diffraction light, the diffraction grating 9 having a cross-section in the shape of a saw blade may be used. In this case, intensity of the +1-order diffraction light is larger than that of the -1-order diffraction light, and, thus, the availability of the reflected optical beam b may be improved. However, it is difficult to manufacture such a diffraction grating 9 having the saw-blade-shaped cross section, and cost of the diffraction grating is increased.
On the other hand, in the second conventional optical system, for generating the focus error signal, both the +1-order diffraction light and the -1-order diffraction light are used. Therefore, as for the availability of the optical beam b reflected on the storage medium, there is no problem as in the first conventional optical system. However, as described later, there is a problem that an offset is caused in a tracking error signal by a beam shift. In general, the tracking error signal is determined by a relationship between positions of the optical beam spot and the track to be scanned by the optical beam spot. A tracking servo operation is carried out based on the tracking error signal. However, when the beam shift occurs, the beam spot is shifted from an optical axis which is a central axis for detecting intensity distribution of the beam spot. Therefore, when the beam shift occurs, even if the track servo regularly operates, the track may not be regularly scanned by the beam spot.
First, a description will be given of a conventional tracking operation of the second conventional optical system when no beam shift occurs, by referring to FIGS. 3A, 3B, 3C to FIGS. 5A. 5B. 5C.
FIG. 3A to FIG. 3C show illustrations for explaining a generating process of the tracking error signal in the second conventional optical system when the beam spot is on the track. FIG. 4A to FIG. 4C show illustrations for explaining the generating process of the tracking error signal in the second conventional optical system when the beam spot is shifted from the track to the right side. FIG. 5A to FIG. 5C show illustrations for explaining a generating process of the tracking error signal in the second conventional optical system when the beam spot is shifted from the track to the left side.
As shown in FIG. 3A, the optical beam is focused on a storage medium 1 to form a small beam spot, for example, a less than 1-.mu.m diameter spot. When the beam spot is diffracted by a track 3 on the storage medium 1, an intensity distribution 21a of a reflected optical beam 21 from the storage medium 1, shown in FIG. 3B, is varied in a vertical plane of an optical axis according to a variation of a location relationship between the beam spot and the track 3. The reflected optical beam 21 is diffracted in the hologram element 13 and is provided to the optical detection system 14 shown in FIG. 2. In FIG. 3C, a relationship the reflected optical beam 21 provided to the hologram element 13 and areas 13-1 to 13-4 of the hologram element 13 are represented.
In the reflected optical beam provided from the area 13-1 of the hologram element 13, the +1-order diffraction light is provided to the optical detector 14a, and the -1-order diffraction light is provided to the optical detector 14c. In the reflected optical beam provided from the area 13-2 of the hologram element 13, the +1-order diffraction light is provided to the optical detector 14b, and the -1-order diffraction light is provided to the optical detector 14d. In the reflected optical beams provided from the areas 13-3, 13-4 of the hologram element 13, the +1-order and -1-order diffraction lights are provided to other optical detector except the optical detectors 14a to 14d, these operational descriptions are eliminated, here.
In those conditions shown in FIG. 3A to FIG. 3C, when output signals of the optical detectors 14a to 14d are referred to as A to D, the tracking error signal TES is represented by the following equation (1). EQU TES=(A+C)-(B+D) (1)
When the beam spot on the storage medium 1 is shifted from the track 3 to the right side of FIG. 3A as shown in FIG. 4A, the intensity distribution 21a of the reflected optical beam 21 from the storage medium 1 is changed as shown in FIG. 4B, and the intensity on the left side of the intensity distribution is increased. Namely, in FIG. 4C, an amount of light provided to the area 13-1 of the hologram element 13 becomes larger than that provided to the area 13-2 thereof. Therefore, an amount of light provided to the optical detectors 14a, 14c becomes larger than that provided to the optical detectors 14b, 14d. As a result, in the condition shown in FIG. 4A, from the equation (1), a relationship TES&gt;0 is established.
On the other hand, When the beam spot on the storage medium 1 is shifted from the track 3 to the left side of FIG. 3A as shown in FIG. 5A, the intensity distribution 21a of the reflected optical beam 21 from the storage medium 1 is changed as shown in FIG. 5B, and the intensity in the right side of the intensity distribution is increased. Namely, in FIG. 5C, an amount of light provided to the area 13-2 of the hologram element 13 becomes larger than that provided to the area 13-1 thereof. Therefore, an amount of light provided to the optical detectors 14b, 14d becomes larger than that provided to the optical detectors 14a, 14c. As a result, in the condition shown in FIG. 5A, from the equation (1), a relationship TES&lt;0 is established.
By the above operation, it is understood that the tracking servo may be carried out by using the tracking error signal TES generated based on the equation (1).
Next, a description will be discussed of the tracking operation when the beam shift occurs, by referring to FIGS. 6A, 6B to FIGS. 8A, 8B. In general, the beam shift is caused by position change of optical components, fluctuation of the plane of the storage medium 1 during rotation, etc. When the beam shift occurs, the central axis of the beam spot is shifted from the optical axis which is the central axis for detecting the intensity distribution of the beam spot.
FIG. 6A and FIG. 6B show illustrations for explaining the generating process of the tracking error signal in the second conventional optical system when no beam shift occurs. FIG. 7A and FIG. 7B show illustrations for explaining the generating process of the tracking error signal in the second conventional optical system when the beam is shifted from the optical axis to the right side by the beam shift. FIG. 8A and FIG. 8B show illustrations for explaining the generating process of the tracking error signal in the second conventional optical system when the beam is shifted from the optical axis to the left side by the beam shift. FIGS. 6A, 7A, 8A respectively show the reflected optical beam 21 provided to the hologram element 13 through the object lens 6, and FIGS. 6B, 7B, 8B respectively show a relationship between the areas 13-1 to 13-4 of the hologram element 13 and the reflected optical beam 21 provided to the hologram element 13.
In FIG. 6A, a central axis 22 of the reflected optical beam 21 is identical to an optical axis 23.
When the beam shift is caused by a position change of the optical components such as an object lens 6, and fluctuation of the plane of the storage medium 1 during rotation, etc., even if the beam spot is regularly formed on the track of the storage medium 1, the central axis 22 of the reflected optical beam 21 may be shifted from the optical axis 23. For example, as shown in FIG. 7A, the central axis 22a is shifted to the right side from the optical axis by a distance d. In this case, a position of the reflected beam 21 on the hologram element 13 is also shifted to the right side as shown in FIG. 7B. Therefore, the amount of light provided to the area 13-2 of the hologram element 13 becomes larger than that provided to the area 13-1 thereof, and, thus, the amount of light provided to the optical detectors 14b, 14d becomes larger than that provided to the detectors 14a, 14c. As a result, the tracking error signal TES is represented as TES=(A+C)-(B+D)&lt;0 by using the equation (1).
In the same way, when the beam shift is caused, even if the beam spot is regularly formed on the track of the storage medium 1, the central axis 22a of the reflected optical beam 21 may be shifted to the left side from the optical axis 23 by the distance d as shown in FIG. 8A. In this case, the position of the reflected beam 21 on the hologram element 13 is also shifted to the left side as shown in FIG. 8B. Therefore, the amount of light provided to the area 13-1 of the hologram element 13 becomes larger than that provided to the area 13-2 thereof, and, thus, the amount of light provided to the optical detectors 14a, 14c becomes larger than that provided to the detectors 14b, 14d. As a result, the tracking error signal TES is represented as TES=(A+C)-(B+D)&gt;0 by using the equation (1).
In this way, when the beam shift is caused, even if the beam spot is regularly formed on the track of the storage medium 1, the tracking error signal TES may not be 0, and may have an offset. When the tracking servo is carried out by using such a tracking error signal TES having the offset, the beam spot may not regularly scan the track on the storage medium 1, so that it may cause a reproduction error.